Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have been increasingly made towards the research and development of batteries. In this aspect, electrochemical devices have attracted the most attention, and particularly, with the recent movement towards minimization and light weight of electronic devices, the development of secondary batteries having a small size and light weight and capable of charging and discharging with high capacity is the focus of particular interest.
A secondary battery is classified, based on a structure of an electrode assembly composed of a positive electrode/a separator/a negative electrode, typically into a jelly-roll (wound) electrode assembly having a structure in which long sheet-type positive and negative electrodes are wound with separators interposed between, and a stack-type electrode assembly having a structure in which a plurality of positive and negative electrodes cut to a predetermined size are stacked in a sequential order with separators interposed between.
However, these traditional electrode assemblies have several problems.
First, a jelly-roll electrode assembly is made by winding long sheet-type positive and negative electrodes with a high density into a cylindrical or oval structure in cross section, and during charge and discharge, stresses caused by expansion and contraction of the electrode accumulate in the electrode assembly, and when the accumulated stresses exceed a predetermined limit, deformation of the electrode assembly occurs. The deformation of the electrode assembly results in non-uniform interval between the electrodes, so the battery performance drastically degrades, and if an internal short circuit occurs, the battery safety may be threatened. Also, when winding long sheet-type positive and negative electrodes, it is difficult to wind the positive and negative electrodes at a high speed with regular intervals therebetween, resulting in reduced productivity.
Second, a stack-type electrode assembly is made by stacking a plurality of positive and negative electrode units in a sequential order, and thus, a delivery process of a polar plate for manufacturing the unit is separately needed and a sequential stacking process takes a lot of time and efforts, resulting in low productivity.
To solve the problems, attempts have been made to develop an electrode assembly of an advanced structure in which a jellyroll type and a stack type are combined, called a stack-folding type electrode assembly having a structure in which bicells or full cells including a predetermined unit of positive and negative electrodes stacked with separators interposed between are folded using a long continuous separator sheet, and examples are disclosed in the Applicant's Patent Application Publication Nos. 2001-0082058A, 2001-0082059, and 2001-0082060.
FIGS. 1 through 3 are cross-sectional views in outline illustrating the structure of a stack-folding type electrode assembly. In the drawings, like reference numerals designate like elements.
Referring to FIGS. 1 through 3, electrode assemblies include a plurality of unit cells 7a, 7b, and 7c1 and 7c2, each unit cell including separators 3a, 3b, and 3c, and negative electrodes 1a, 1b, and 1c and positive electrodes 5a, 5b, and 5c disposed at both sides of the separators 3a, 3b, and 3c, respectively. The positive electrodes 5a, 5b, and 5c have a structure in which a positive electrode active material layer is formed on both surfaces of a positive electrode current collector, and the negative electrodes 1a, 1b, and 1c have a structure in which a negative electrode active material layer is formed on both surfaces of a negative electrode current collector. As shown in FIGS. 1 through 3, the unit cell may be formed in various structures including a structure of full cells 7a and 7b in which the positive electrode 5a and 5b and the negative electrode 1a and 1b are disposed one by one at both sides of the separator 3a and 3b, and a structure of bicells 7c1 and 7c2 in which each separator 3c is disposed on both surfaces of the positive electrode 5c or the negative electrode 1c and the negative electrode 1c or the positive electrode 5c is each disposed on each separator 3c (a structure of positive electrode/separator/negative electrode/separator/positive electrode or a structure of negative electrode/separator/positive electrode/separator/negative electrode).
Inside the electrode assemblies 10, 20, and 30, each of the unit cells 7a, 7b, 7c1, and 7c2 exists in a stacked shape. In this instance, continuous single folding separators 9a, 9b, and 9c are arranged in various shapes as shown in FIGS. 1 through 3 between the adjacent unit cells 7a, 7b, and 7c1 and 7c2 corresponding to each other, surrounding the unit cells 7a, 7b, and 7c1 and 7c2, respectively, and perform a separator function between the unit cells 7a, 7b, and 7c1 and 7c2, respectively.
The manufactured stack-folding type electrode assembly is received in a battery case, followed by injection of an electrolyte solution, to fabricate a battery. However, after injecting the electrolyte solution, the time for the electrolyte solution to permeate the separator sufficiently is needed. Due to the productivity problem, however, a sufficient impregnation time is not given in real situation, disallowing the electrolyte solution to permeate the separator, resulting in low wettability, and the electrolyte solution that did not permeate the separator may leak in severe conditions. Therefore, there is a need for approaches to allow easy impregnation of the electrolyte solution in the stack-folding electrode assembly.
In addition, because the stack-folding electrode assembly surrounds the unit cells with the folding separator, the electrode assembly inevitably increases in thickness according to the structure due to the folding separator. In keeping up with the recent trend towards volume minimization of secondary batteries, there is the demand for approaches to reduce the volume of the electrode assembly.